The present invention is concerned with a method for producing a reinforced polymer by introducing carbon nanotubes into the polymer. The invention also relates to reinforced polymers produced by the present methods and the use of carbon nanotubes for improving the mechanical properties of polymers.
It has been known for many years that blending fibres, such as carbon fibres, with polymers can significantly improve the mechanical properties of the blends (see Polymer Composites, April 1987, Vol. 8, No. 2, 74-81; J. Composite Materials, Vol. 3, October 1969, 732-734; and Polymer Engineering and Science, January 1971, Vol. 11, No. 1, 51-56). GB 1179569A discloses a method of reinforcing polymers by the incorporation of long fibres of material such as metal, glass or asbestos. The advantage of carbon fibres is that they are very light, but despite this exhibit relatively great mechanical strength. In particular they exhibit very high stiffness.
More recently, since the discovery of Buckminsterfullerene (C.sub.60), it has been found that carbon tubes (often termed carbon nanotubes because of their diminutive dimensions) having a structure related to the structure of C.sub.60 exist, which have the potential to be used in similar ways to carbon fibres. In particular, the structure of carbon nanotubes makes their aspect ratio (length/diameter, L/D) comparable to that of long fibres. Typically the aspect ratio of carbon nanotubes is &gt;10000. Thus, the aspect ratio of carbon nanotubes is generally much greater than that of conventional short fibres, such as short glass fibres and short carbon fibres. In addition, the tubes can potentially be lighter than conventional carbon fibres, whilst being stronger and stiffer than the best conventional carbon fibres (see P. Calvert "Potential application of nanotubes" in Carbon Nanotubes, Editor T. W. Ebbeson, 297, CRC, Boca Raton, Fla. 1997).
Depending on their diameter, helicity, and number of layers (single-wall v. multiple-wall) carbon nanotubes have electronic properties between those of conductors and semi-conductors. They may thus be added to an electrically insulating polymer to increase its conductivity. WO 97/15934 discloses an electrically conductive polymer composition containing carbon nanotubes. In addition, carbon nanotubes have great mechanical strength, being cited as having bending modulus values of from 1000-5000 GPa. Moreover they have been mentioned in connection with new, highly efficient, fracture micromechanisms which would prevent pure brittle failure with a concomitant low strain. Thus, carbon nanotubes have been evisaged for use in many applications in recent years (see P. Calvert "Potential application of nanotubes" in Carbon Nanotubes, Editor T. W. Ebbeson, 297, CRC, Boca Raton, Fla. 1997; T. W. Ebbeson, "Carbon Nanotubes", Annu. Rev. Mater. Sci., 24, 235, 1994; Robert F. Service, "Super strong nanotubes show they are smart too", Science, 281, 940, 1998; and B. I. Yakobson and R. E. Smalley, "Une technologie pour le troisieme millenaire: les nanotubes", La Recherche, 307, 50, 1998).
However, in the past when producing polyolefin composites by incorporating carbon nanotubes, tangling of the nanotubes and consequent randomising of the orientations of the nanotubes has caused problems (see M. S. P. Shaffer, X. Fan, A. H. Windle, "Dispersion of carbon nanotubes: polymeric analogies", poster 39, p. 317 in Proceedings of Polymer '98", September 1998, Brighton (UK); P. M. Ajayan, "Aligned carbon nanotubes in thin polymer films", Adv. Mater., 7, 489, 1995; H. D. Wagner, O. Lourie, Y. Feldman and R. Tenne, "Stress-induced fragmentation of multi-wall carbon nanotubes in a polymer matrix", Appl. Phys. Lett., 72 (2), 188, 1998; and K. Yase, N. Tanigaki, M. Kyotani, M. Yomura, K. Uchida, S. Oshima, Y. Kuriki and F. Ikazaki, Mat. Res. Soc. Symp. Proc., Vol. 359, 81, 1995). In particular, tangling can give rise to a reduction in the homogeneity of fibre/polymer blends since it is difficult for the fibres to distribute themselves evenly within the surrounding polymer matrix. This reduces the mechanical strength of the blends, since lack of homogeneity introduces weak points in a blend at positions where, for instance, there is a relatively low concentration of fibre and a high concentration of polymer. Moreover the randomising of the orientation of the fibres also reduces the mechanical strength of the blends. This is because (for example) the maximum resistance to strain in a given direction will be achieved when all of the fibres in the blend are oriented with their longitudinal axes aligned in that direction. The further that a blend deviates from such an ideal orientation, the less the resistance to strain of the blend in that direction. However, up to present it has not been possible to control the orientation of the fibres to a degree sufficient to improve mechanical properties.